8.13: Wind Waves

If ocean waves are caused by wind, how can there be strong waves on calm days?

Waves form where there are winds. Energy from the wind is transferred to the water and then that is transferred to nearby water molecules. The wave moves as a transfer of energy across the sea. Once the wave starts, it doesn't need more wind to keep it going.

Ocean Waves

Waves
have been discussed in previous concepts and lessons in several contexts: seismic waves traveling through the planet, sound waves traveling through seawater, and ocean waves eroding beaches. Waves transfer energy, and the size of a wave and the distance it travels depends on the amount of energy that it carries. This lesson studies the most familiar waves, those on the ocean's surface.

Building Big Waves

Ocean waves originate from wind blowing – steady winds or high storm winds – over the water. Sometimes these winds are far from where the ocean waves are seen. What factors create the largest ocean waves?

The wind could be strong, but if it gusts for just a short time, large waves won’t form.

Wind blowing across the water transfers energy to that water. The energy first creates tiny ripples, which make an uneven surface for the wind to catch so that it may create larger waves. These waves travel across the ocean out of the area where the wind is blowing.

Remember that a wave is a transfer of energy. Do you think the same molecules of water that start out in a wave in the middle of the ocean later arrive at the shore? The molecules are not the same, but the energy is transferred across the ocean.

Shape of a Wave

Water molecules in waves make circles or ellipses (
Figure
below
). Energy transfers between molecules, but the molecules themselves mostly bob up and down in place.

The circles show the motion of a water molecule in a wind wave. Wave energy is greatest at the surface and decreases with depth. "A" shows that a water molecule travels in a circular motion in deep water. "B" shows that molecules in shallow water travel in an elliptical path because of the ocean bottom.

Waves Break

When does a wave break? Do waves only break when they reach shore? Waves break when they become too tall to be supported by their base. This can happen at sea but happens predictably as a wave moves up a shore. The energy at the bottom of the wave is lost by friction with the ground, so that the bottom of the wave slows down but the top of the wave continues at the same speed. The crest falls over and crashes down.

Storm Surge

Some of the damage done by storms is from
storm surge
. Water piles up at a shoreline as storm winds push waves into the coast. Storm surge may raise sea level as much as 7.5 m (25 ft), which can be devastating in a shallow land area when winds, waves, and rain are intense.